This invention relates in general to a thread cutting device and in particular to thread cutting devices leaving uniform length ends on zig zag sewing machines.
A similar thread cutting device is known from West German Utility Patent DE-GM No. 1,968,920. In this thread cutting device, the thread cutter, cooperating with the cutting edge of the thread take-up is attached to the lower side of the needle plate at a location spaced laterally from the stitch hole, of oblong shape.
Since the needle has a greater distance from the stationary thread cutter during zigzag sewing in overstitch position than in the other, there will be thread ends of different lengths on the fabric sewn after the thread is cut; the shortest thread ends will be formed in the overstitch position facing the thread cutter, and the longest thread ends will be in the overstitch position facing away from it.
West German Patent Specification DE-PS No. 31,24,795 discloses another thread cutting device for zigzag sewing machines, in which the thread take-up has, on its site opposite the barb, a second barb for the needle thread loop leg leading to the needle in order to make the length of the needle thread ends connected to the needle and the thread reservoir independent of the stitching position of the needle. However, the arrangement of the second barb does not affect the length of the thread ends on the fabric being sewn, i.e., their length depends on the last stitching position, just as in the first thread cutting device.
Finally, West German Patent Specification No. 17,60,703 discloses a thread cutting device for double chain stitch sewing machines, which has a thread take-up that can be moved in parallel to the plane of the needle plate and has a cutting edge, as well as a thread cutter that is arranged in parallel to this and can be moved in the same direction, and a thread clamp that can be moved together with the thread cutter. The thread take-up is provided with a lug that engages with one elongated slot of the thread cutter and one thread clamp each, such that the thread cutter and the thread clamp move together with the thread take-up from the resting position into the work position and vice versa whenever the pin has traveled over the entire length of the elongated slots. To prevent the thread cutter and the thread clamp from moving prematurely and in an uncontrolled fashion, they are held in their two end positions by a spring-loaded ratchet mechanism.
Aside from the fact that this thread cutting device was not designed for double lock stitch but for double chain stitch sewing machines, which are used for straight sewing rather than for zigzag sewing, it is also unsuitable for cutting through thicker threads of greater tear strength, which offer such a great resistance to cutting that the force to be applied for cutting is greater than the holding force of the spring-loaded ratchet mechanism, because in this case, the thread cutter would be forced out of the actually intended cutting position located in the immediate vicinity of the stitch hole before the threads are cut through completely, and the cutting process would be completed only in the starting position of the thread cutter located relatively far from the stitch hole. However, undesirably long thread ends would thus be obtained on the fabric sewn.